Cosmetic preparations, such as creams or lotions, comprising UV filters are used to protect the skin from too intensive UV radiation. As UV filters these preparations generally comprise one or more organic compounds which absorb in the wavelength range from 290 to 400 nm, which is divided into the ranges: UVB radiation (290 to 320 nm); UVA radiation (320 to 400 nm).
UVB radiation, which is higher-energy, causes the typical symptoms of sunburn and also causes suppression of the immune defence, while UVA radiation, which penetrates more deeply into the layers of the skin, causes premature ageing of the skin. Because the cooperation of the two types of radiation is said to promote the development of skin diseases caused by light, such as skin cancer, there has been a search for possible ways of further improving significantly UV protection.
Metal oxides, such as titanium dioxide or zinc oxide, are widely used in sun protection agents. Their action is based substantially on the reflection, scattering and absorption of the damaging UV radiation and is dependent substantially on the primary particle size of the metal oxides. Microfine titanium dioxide is used in many cosmetic formulations because it is chemically inert and toxicologically harmless and results neither in skin irritations nor sensitisation. Microfine titanium dioxide can be prepared by wet chemical precipitation processes or thermal (pyrogenic) gas-phase processes.
Pyrogenically prepared silicon dioxide-titanium dioxide mixed oxide is known from DE 4235996. A disadvantage of the known pyrogenically prepared silicon dioxide-titanium dioxide mixed oxide is its photocatalytic activity, which initiates reactions that can lead to a change in constituents of a sun protection agent. Numerous methods have therefore been developed for lowering the photocatalytic activity of these metal oxides without reducing their UV-screening properties, for example by surrounding them with a shell of silicon dioxide and/or aluminium oxide.
The photoactivity of titanium dioxides can also be reduced by rendering them hydrophobic with organosilanes. A microfine, pyrogenic titanium dioxide that has been established on the market for many years is titanium dioxide T 805 or AEROXIDE TiO2 T 805 (Degussa AG). This product can be prepared by the flame hydrolysis of titanium tetrachloride according to the AEROSIL process and subsequently rendering the product hydrophobic with an organosilane. Pyrogenic titanium-iron mixed oxides that have been rendered hydrophobic are described, for example, in EP 0722992.
However, hydrophobic, pyrogenic titanium dioxides cannot be dispersed in the water phase of a sun protection emulsion but only in the oil phase. However, because the water phase generally accounts for more than 60%, the larger part of a sun protection formulation cannot be used as “carrier” for titanium dioxide. Consequently, the titanium dioxide content of a sun protection emulsion has an upper limit and is generally only from 3 to 5 wt. %, based on the total formulation. This means, that without the addition of further organic UV filters and/or zinc oxides, sun protection agents having a light protection factor >10 are difficult to produce.
Surface-treated hydrophilic, pyrogenic titanium dioxides can be prepared by coating them with silicon dioxide, as described, for example, in DE 10260718 and further publications mentioned therein. However, such materials exhibit a pronounced photoactivity and therefore have only limited suitability for use as UV filters.
Further disadvantages of known hydrophobic and hydrophilic, pyrogenic titanium dioxides are:    insufficient transparency of the sun protection formulations prepared therewith on application to the skin    complicated dispersion is necessary    a pronounced thickening effect on dispersion in, for example, cosmetic oils or water makes it difficult to prepare dispersions or sun protection agents having a high TiO2 content    the sun protection formulations that are prepared feel dull on the skin.
Accordingly, one object in this area is to find a pyrogenically prepared silicon dioxide-titanium dioxide mixed oxide that does not exhibit these disadvantages. A further object to prepare sun protection agents that have improved transparency and sensory properties and that have a high sun protection factor.